Subsequent Events	3 Months Ended
Mar. 31, 2014
Subsequent Event [Line Items]
Subsequent Events	Subsequent Events In April 2014, the Company received notification that $61.1 million principal amount of the Company's 1.875% convertible senior notes were surrendered for conversion, and settlement is anticipated during the second quarter of 2014. In accordance with the terms of the notes, the Company elected to settle these conversion obligations through a combination of cash, in an amount up to the principal amount of the converted notes, and shares in respect of any excess thereof (based on the average of the volume-weighted-average prices of the Common Stock during the 40 trading-day cash settlement averaging period specified in the indenture governing the notes). In connection with these note conversions, the Company exercised a proportionate amount of its convertible note hedges, for which the Company expects to receive shares of Common Stock equivalent to the number of shares the Company will be required to issue to settle the non-cash portion of the related note conversions. In May 2014, the Company entered into a research collaboration and license agreement with Avalanche Biotechnologies, Inc., a privately held company, to discover, develop, and commercialize novel gene therapy products for the treatment of ophthalmologic diseases. In connection with the agreement, the Company is required to make a $2.0 million upfront payment and a $6.0 million pre-payment of collaboration research costs, and is obligated to pay potential additional research costs, potential development and regulatory milestones (for products directed to as many as eight therapeutic targets), and royalties on any future sales of such products. The Company has also purchased an aggregate of $5.0 million of Avalanche preferred stock. Under the agreement, the Company will collaborate with Avalanche to conduct research for the discovery of novel gene therapy vectors. Subsequent to the filing of an Investigational New Drug application ("IND") with the FDA for a product candidate, Regeneron may exercise its right to obtain exclusive worldwide rights to further research, develop, and commercialize such product candidates directed to the applicable therapeutic target. In addition, Avalanche has the option to share in development costs and profits for products directed toward up to two therapeutic targets of its choice.

Subsequent Events

In April 2014, the Company received notification that $61.1 million principal amount of the Company's 1.875% convertible senior notes were surrendered for conversion, and settlement is anticipated during the second quarter of 2014. In accordance with the terms of the notes, the Company elected to settle these conversion obligations through a combination of cash, in an amount up to the principal amount of the converted notes, and shares in respect of any excess thereof (based on the average of the volume-weighted-average prices of the Common Stock during the 40 trading-day cash settlement averaging period specified in the indenture governing the notes). In connection with these note conversions, the Company exercised a proportionate amount of its convertible note hedges, for which the Company expects to receive shares of Common Stock equivalent to the number of shares the Company will be required to issue to settle the non-cash portion of the related note conversions.

In May 2014, the Company entered into a research collaboration and license agreement with Avalanche Biotechnologies, Inc., a privately held company, to discover, develop, and commercialize novel gene therapy products for the treatment of ophthalmologic diseases. In connection with the agreement, the Company is required to make a $2.0 million upfront payment and a $6.0 million pre-payment of collaboration research costs, and is obligated to pay potential additional research costs, potential development and regulatory milestones (for products directed to as many as eight therapeutic targets), and royalties on any future sales of such products. The Company has also purchased an aggregate of $5.0 million of Avalanche preferred stock. Under the agreement, the Company will collaborate with Avalanche to conduct research for the discovery of novel gene therapy vectors. Subsequent to the filing of an Investigational New Drug application ("IND") with the FDA for a product candidate, Regeneron may exercise its right to obtain exclusive worldwide rights to further research, develop, and commercialize such product candidates directed to the applicable therapeutic target. In addition, Avalanche has the option to share in development costs and profits for products directed toward up to two therapeutic targets of its choice.